1. Field of the Invention
The present invention relates to a coating method of forming an insulating film by coating a coating liquid dispersed therein particles or colloids destined to be a starting material of a film component in a solvent on a surface of a substrate, and relates to a coating unit, an aging unit, and a solvent replacement unit, and a film coating apparatus for coating a film.
2. Description of the Related Art
As a method of forming an interlayer insulating film of a semiconductor device, CVD and thermal oxidation are well known. On the other hand, other than these, there is a method called a sol-gel method. In this sol-gel method, a coating liquid dispersed colloids of TEOS (tetra-ethoxy-silane; (Si(C.sub.2 H.sub.5 O).sub.4) in an organic solvent such as ethanol solution is coated on a surface of a semiconductor wafer(hereinafter, simply refers to as wafer), the coated film, after being gelled, is dried, to obtain a silicon oxide film. The examples of this sol-gel method are disclosed in Japanese Patent Laid-Open Application Nos. HEI-8-162450 and HEI-8-59362.
Behavior in the course of change of the coated film in this sol-gel method is shown schematically in FIG. 10A through FIG. 10C. First, particles or colloids 100 of the TEOS, when coated on a wafer, are in a dispersed state in a solvent 200 (FIG. 10A). Then, this coated film, by being subjected to an basic atmosphere or to heating, is promoted in polycondensation and hydrolysis of the TEOS. As the result, the coated film is gelled to form a reticular structure of the TEOS 300(FIG. 10B).
Next, in order to remove moisture in the coating liquid, the solvent in the coated film is replaced by another solvent 400 of which boiling point is low and surface tension is small(FIG. 10C). By drying further thereafter, a coated film of a silicon oxide film is obtained.
Incidentally, a step of replacing the solvent shown in FIG. 10C is carried out to remove moisture. Further, the step of replacing with the solvent is carried out with an object to make the film hydrophobic. That is, since OH group is liable to absorb the moisture, the OH group combined to a terminal portion of Si--O bond is replaced by another organic substance by cleaning the film with, for instance, HMDS or the like.
Further, another object is to suppress collapse of a structure of the film by employing a solvent of smaller surface tension than ethanol so that a large force is not placed on the reticular structure of the TEOS when the solvent vaporizes.
Thus, there are an uncountable number of minute pores in the silicon oxide film formed by the sol-gel method, there comes in air. Therefore, relative dielectric constant of the oxide film .epsilon. is close to that of air. Therefore, the electric resistance of the oxide film including such minute pores becomes such high as is close to that of air, resulting in an ideal insulating film.
In order to apply such a sol-gel method in an actual manufacturing line, a coating unit of coating a coating liquid on a wafer, an aging unit of gelling the coated film by heating the wafer at a pre-determined temperature, for instance, at 100.degree. C., and a replacement unit of replacing the solvent in the coated film by another solvent are necessary. Further, a pre-treatment unit for carrying out the pre-treatment such as hydrophobic treatment to the wafer, and a baking unit of drying the wafer are also necessary. And, by disposing a transfer mechanism of transferring the wafers between respective units, an apparatus is constituted.
Now, upon carrying out gelling treatment of the coated film, it is necessary that, through suppression of evaporation of the solvent from the coated film, gelling of the TEOS is made not to be disturbed. For this, it is not desirable to carry out gelling operation in an open system like the conventional method. Instead, the gelling treatment needs only to be carried out in a sealed container of an atmosphere filled with vapor of the solvent. That is, after disposing a wafer on a stage heated at a pre-determined temperature in the sealed container, gas including vapor of the solvent needs only to be introduced.
Now, the inventors of the present invention investigated to carry out the gelling operation with an aging unit 3 of a structure illustrated in FIG. 11, for instance. The aging unit 3 comprises a heating plate 30, a cover 33 provided with a heater, a gas introducing path 34, and a gas exhausting path 35.
As shown in FIG. 11, in this aging unit 3, the heating plate 30 is formed of ceramic and incorporates a heater 31a.
The cover 33 is provided with a heater. This cover 33 is intimately connected to a circumference portion of the heating plate 30 through a sealing member 32, and, together with the heating plate 30, constitutes a sealed container defining a treatment chamber S.
In the heating plate 30, outside along the circumference portion of the wafer W, a gas introducing inlet 34a is formed in slit. This gas introducing inlet 34a penetrates the heating plate 30 and is connected to a gas introducing path 34 introducing gas from the outsides.
At the central portion of the cover 33, an exhausting outlet 35a is disposed. This exhausting outlet 35a penetrates the cover 33 to connect the exhausting path 35 communicating with the outsides. In addition, to the aging unit 3, 3 pieces of pins 36 for going up and down are disposed, for instance, and with these pins for going up and down, the wafer W is moved up and down between the heating plate 30 and the above position thereof.
As a gas to be employed here, a gas of a solvent component, for instance, vapor of ethylene glycol, is employed in order to suppress evaporation of the solvent inside the coated film during the atmosphere for heat treatment is heated.
And the gas introducing path 34 is disposed penetrating the heating plate 30. Therefore, the gas is introduced into the sealed container after being heated to approximately 100.degree. C., for instance, or a temperature extremely close to that temperature. At this time, the concentration of the vapor of ethylene glycol is a concentration that is equal with the saturated vapor pressure at a treatment temperature of approximately 100.degree. C., for instance. Further, in addition to ethylene glycol, a gas accelerating gelation, ammonia gas, for instance, can be introduced simultaneously.
However, in such a sealed container as the aging unit 3 shown in FIG. 11, when introduction of gas therein is started, there is such a problem that until supply of the gas reaches a steady state, the concentration and temperature of the gas fluctuate.
Also as aforementioned, in the case of a wafer being processed at a pre-determined temperature higher than normal temperature, 100.degree. C., for instance, when the wafer of the room temperature is transferred as it is into the sealed container of a pre-determined temperature, the gas of the solvent component which is at the saturated vapor pressure in the sealed container contacts with the wafer, to lower the temperature thereof. As the result, it becomes a super-saturated state, and the gas of the solvent component is likely to condense. When ethylene glycol, for instance, condenses on the wafer, the thickness of that part becomes thick, resulting in a problem of becoming a non-homogeneous insulating film.